(1) Field of the Invention
The present invention relates to a method for operating a converter used for steel refining.
(2) Description of the Prior Art
In refining molten pig iron and steel in a converter, pure oxygen is ejected from a lance inserted through the mouth of the converter into the converter body (below "vessel"). The oxygen is blown onto the molten steel to both effect decarburization and stir the molten steel. In addition, flux is charged into the converter to slag the flux and hence to form molten slag, thereby effecting dephosphorization, desulfurization, or the like due to the reactions between the molten slag and steel.
Slag foaming occurs due to several slag conditions, such as the slag composition, viscosity,the total amount of oxygen in the slag, etc. Too extensive slag foaming causes the slag and even molten steel to overflow the converter mouth, which overflow is referred to as "slopping". Of course, the composition of the molten steel and the steel yield are greatly influenced by slopping. Also, various problems are caused, such as reduction in the operational efficiency and in th calorific content of the recovered gases, impairment of the operational environment, e.g., generation of brown smoke, and damage to the steelmaking devices. Slopping therefore must be suppressed as much as possible.
Various proposals have been made on how to enable prompt prediction of the slag conditions within a converter and hence realize optional converter operation without slopping.
Japanese Unexamined Patent Publication (Kokai) No. 52-101618 discloses a method for estimating the amount of slag by calculating the oxygen balance based on information on the waste gases during blowing and then estimating the amount of oxides formed in the converter, i.e., the molten slag. In this method, however, there is an unavoidable time delay due to the gas analysis and mathematical analysis. In addition, since slopping is not dependent upon just the amount of molten slag alone, the accuracy of prediction of slopping is not very high.
Various attempts have also been made on detecting the slag level by physical means. These include an acoustic measuring method (Japanese Unexamined Patent Publication No. 54-33790), a vibration measuring method (Japanese Unexamined Patent Publication No. 54-114,414), a method for measuring the inner pressure of a coverter (Japanese Unexamined Patent Publication No. 55-104,417), a method usig a microwave gauge (Japanese Unexamined Patent Publication No. 57-140812), and a method for measuring the surface temperature of the converter body (Japanese Unexamined Patent Publication No. 58-48615).
In the acoustic measuring method, changes in the frequency and magnitude of the acoustics generated in the converter are monitored to estimate the slag level and to predict slopping.
In the vibration measuring method, changes in the magnitude of lance vibration and the wave transition of the lance vibration are monitored during blowing to estimate the slag level or conditions and then to predict slopping.
In the method for measuring the inner pressure of a converter, variations in the ejecting pressure of the waste gases through the converter mouth are monitored to predict slopping.
In the method using a microwave gauge, a microwave is directly projected into the converter interior to directly measure the slag level based on the FM radar technique ad to predict slopping.
In the method for measuring the surface temperature of a converter body, the energy emission from the upper and lower parts of the converter body is detected as temperature, and the occurrence and magnitude of slopping are predicted based on the temperature magnitude and peak values.
The acoustic measuring method, vibration measuring method, method for measuring the inner pressure of a converter, and method for measuing the surface temperature of the converter body are all indirect measuring methods and suffer from low accuracies of prediction of slopping due to the inability to quantitatively measure the slag level or conditions. The method using a microwave gauge enables direct measurement of the slag level, but suffers from the fact that it is not easy to detect or estimate abnormalities by microwave measuremnt, since the melt, slag, gases, and the like effect consideraly complicated movement in the converter during blowing. In addition, this method requires sophisticated signal processing, which increases the cost of the measuring device.
Three of the present inventors studied the foaming behavior of slag and discovered that the light intensity and/or wave length of the gaseous atmosphere and the wavelength characteristics of light emitted from the gaseous atmosphere considerably differ from those of the slag. The above three inventors provided, in Japanese Patent Application No. 58-37872, a method for directly observing slag-forming conditions, i.e., the slag-foaming conditions, in a converter during blowing, characterized in that at least one observation device of the vessel-interior light is disposed in at least one throughhole of the side wall of a converter so as to face the vassel interior and observe the slag-forming conditions.